1. Field of the Invention
This invention relates to the separation of gases. More particularly, it relates to purification of a hydrogen-rich stream by the removal of acid gases, CO2, H2S, and COS, by the method of autorefrigeration. Further, this invention relates to a method for avoiding the emission to the atmosphere of CO2, a so-called greenhouse gas. In a highly preferred embodiment, it relates to the production of CO2 at a pressure sufficiently high for disposal by containment, a method commonly known as sequestration. The sequestration method itself is not a part of this invention.
2. Description of the Prior Art
There is increasing concern about combustion of fossil fuels worldwide because of the emission of carbon dioxide. Atmospheric CO2 is believed capable of producing a “greenhouse effect” by trapping radiated heat from the earth's surface, thereby contributing to global warming. Although emission of CO2 to the atmosphere is not yet regulated, the issue is one of such rising political concern that future regulation is a strong possibility and worthy of new technology and invention to address the problem. It has been proposed in many technological forums that a way to limit the emission of CO2 from fossil fuels is to utilize the energy in the fossil fuel to make hydrogen, which emits only water vapor when combusted. During hydrogen production, the carbon in the fossil fuel is converted to CO2. Under current proposals, the CO2 is then separated from the hydrogen and compressed to a high pressure for disposal. The high pressure is necessary for carrying out the most commonly proposed method of disposal: sequestration by deep underground or deep ocean containment. Although many commercial processes are available to produce purified hydrogen and CO2, the energy consumed by undertaking both the separation process and the CO2 compression process is quite high, making current processes economically unattractive. Our invention proposes a process to greatly decrease this energy consumption.
The processes for making hydrogen from fossil fuels are well-known. One broad class of these processes is gasification, in which a carbonaceous fuel (e.g., coal) is partially oxidized at high temperature and elevated pressure in the presence of water vapor to form mainly carbon monoxide (CO) and hydrogen (H2). Then by the well known water-gas shift conversion reaction, the carbon monoxide is reacted with water vapor over a catalyst to form additional hydrogen and carbon dioxide. Sulfur in the fossil fuel is converted mainly to hydrogen sulfide during gasification. The hydrogen is then purified to remove CO2 and H2S by a well known process method commonly called acid gas removal (so named because the compounds CO2 and H2S will ionize in water to form mildly acidic solutions).
There are numerous methods for acid gas removal. Most commercially-applied processes use some form of solvent that has an affinity for acid gases. The solvents vary broadly and include chemical substances such as monoethanolamine in water, chilled methanol, or hot potassium carbonate ionized in water. The reference book Gas Purification, fifth edition, lists more than a dozen solvent-based processes for acid gas removal. Typically, the acid gases are absorbed into the solvent in an absorption tower to form a solvent stream rich in acid gases. Acid gases are then removed from the rich solvent by some combination of flashing at reduced pressure, stripping with a medium of nitrogen or steam, and/or distillation of the solvent. The solvent, now lean with respect to acid gases, is then returned to the absorption tower.
A chief drawback to these solvent-based acid gas removal processes is that a significant quantity of energy, either in the form of steam or electricity, is required to regenerate the solvent. The very act of diluting the acid gases within a solvent means that significant energy is required to reconstitute the acid gases as a pure stream. This energy penalty is made worse if the acid gases must be pressurized for sequestration. The pressure lost during flashing of the solvent at a reduced pressure must then be restored by compression of the acid gases. Even further energy must be expended if the H2S must be separated from the CO2 prior to sequestering the CO2 (an issue which has yet to be settled by environmental regulation).
Our invention uses the well-known method of autorefrigeration to remove acid gases. With autorefrigeration, acid gases are condensed and separated from the hydrogen stream, and the condensed acid gas itself is used as the refrigerant for cooling. Autorefrigeration is a standard method found in the prior art for purifying many types of gases, including hydrogen. In general, however, where these patented processes differ from our invention, is that our invention uses a series of autorefrigeration stages to remove and capture acid gases at multiple pressure levels, thereby greatly reducing the energy needed to pressurize the acid gases to the desired pressure for sequestration.
Autorefrigeration has been previously patented as a method of acid gas removal. In U.S. Pat. No. 3,001,373, Eastman et al., 1961, a process is described in which the bulk portion of CO2 is removed by autorefrigeration, followed by the use of a chilled solvent to absorb additional CO2. The solvent is regenerated by stripping with air at atmospheric pressure. The portion of CO2 removed from the solvent in this latter manner is diluted by air and thus is not suitable for recovery for later sequestration. The pure CO2 portion collected during autorefrigeration is expanded to near atmospheric pressure to generate additional cooling for the process, thereby making this portion of the CO2 less suitable for sequestration because of the high energy requirement needed for pressurization.
In U.S. Pat. No. 3,614,872, Tassoney et al., 1971, a process is described in which hydrogen is purified of acid gases in a single stage of autorefrigeration. The autorefrigeration step is carried out by condensing acid gases at a temperature a few degrees above the freezing point of CO2 (about −70° F.). Our invention is quite similar in that it uses the same basic autorefrigeration process step. However, our invention is an advance over Tassoney in two ways. First, our invention employs more than one autorefrigeration stage, which greatly reduces the energy requirements for pressurization of the acid gas stream. Second our invention employs a novel concept of using an antifreeze liquid compound to permit autorefrigeration to take place at a temperature colder than the normal freezing point of CO2. This allows more acid gas to be removed from the hydrogen stream than would be possible using Tassoney's process.